1. Field of the Invention
The present invention relates to methods and apparatus for treating sludge, and in particular to mixing apparatus and transportation methods.
2. Description of Related Art
Plants such as wastewater treatment plants produce a sludge that normally requires processing before disposal or use as a fertilizer. The sludge has a high level of pathogens such as hazardous bacteria and viruses. These pathogens need to be eliminated before using the sludge as a fertilizer. For this reason, many wastewater treatment plants employ a digester, a large vessel where benign bacteria is released to spread and destroy the pathogens.
In some circumstances, instead of bacteria, an alkaline substance such as lime is mixed with the sludge to destroy pathogens. Current regulations require initially raising the pH of the sludge to at least 12 so as to keep the pH at least 12 for at least 2 hours without the need to add more alkaline substance. On the other hand, where the sludge is used as a fertilizer, excessive alkalinity may be unacceptable because of the tendency to harm vegetation. Also, excessive alkalinity can dry the sludge and cause caking, making the sludge difficult to handle.
Some treatment plants do not have the ability or capacity to digest the sludge or treat it with lime. Bacterial digestion of sludge may take more than 20 days and a facility may lack adequate capacity for keeping sludge for such an extended time period. In such cases untreated or partially treated sludge must be shipped and possibly treated elsewhere. Shipping sludge that has not been fully treated requires special precautions. Also, this sludge is putrescent and its odor can draw objections from any vicinity through which it passes or arrives.
Upon arrival at a destination, treatment of the sludge typically requires several time-consuming and costly steps. The sludge is often unloaded, transferred to a pug mill where lime is mixed with the sludge, unloaded from the mill, placed in the windrow for at least 24 hours to complete the stabilization, and then carried to a site to be spread as fertilizer. Besides the high costs associated with handling sludge at the destination, there are even higher costs associated with getting permits for mixing/stabilizing the untreated or partially treated sludge at the destination. In many cases obtaining permits will be so difficult as to make treatment at the destination impractical.
U.S. Pat. No. 4,925,571 concerns a method for heat pasteurization of sewage sludge during shipment by rail, truck, ship, etc. Waste heat from the vehicle propulsion system is used for the heating. The pasteurized sludge is eventually used as fertilizer. The process requires that the sludge be heated to a specific temperature for a specific period of time sufficient to reduce the presence of pathogens within the sludge, but not induce dewatering of the material.
U.S. Pat. No. 5,971,600 concerns a mobile trailer for combining waste material (sewage sludge or other biological waste) with a stabilizing material such as lime. Waste material placed in a reservoir, and stabilizer placed in another reservoir are fed to a plow blender for mixing the waste material and stabilizer. The mixture is then discharged from the trailer.
U.S. Pat. No. 5,441,640 concerns a transportable plant for treating biological waste in a manner much like a municipal-type treatment process. The plant employs bacteria to digest waste material in a holding tank which contains an oxygen injection system. Pumps are used to draw water from one tank to the next and to draw wastewater into the wastewater holding tank.
U.S. Pat. No. 5,490,741 concerns a mobile apparatus for the metered blending of sludge or other material with dry or liquid stabilizing compounds. The particle size of introduced biological sludge material is adjusted by means of vibratory screens and/or flailing chains prior to entering a retention bin. The sludge is then combined with a stabilizing element within a mixing chamber. The mixing chamber contains a mixing element such as an auger-type screw apparatus which both mixes the sludge and lime and moves the material through to a discharge opening.
U.S. Pat. No. 3,772,188 concerns the pretreatment of sewage or other biological waste. An intermediate treatment process occurs in a conduit where raw material is being piped from a source of origin to point of treatment. The apparatus consists of a pressurized vessel containing a lower portion, which holds the standing sewage, and an upper portion where the standing sewage is injected and where it is subjected to processes, which both break-up and aerate the sewage particles. This process is meant to expose the sewage particles to high purity oxygen, which is pumped into the vessel under pressure. The reference discusses treating the pH of the effluent before discharge.
U.S. Pat. No. 5,013,458 concerns a device and method for reducing the presence of pathogens within a biological waste substance, such as sewage sludge. In this reference, a chemical reagent such as lime is mixed with the sludge material in order to raise the temperature and pH of the material, which in turn, kills the pathogens. In order to properly blend the chemicals and waste material, the material is passed through a narrow chamber by means of a helically shaped screw.
In U.S. Pat. No. 5,681,481 liquid sludge and lime or other caustic material are thoroughly mixed within a container to increase the pH of the sludge to a desired level and hold the material at that pH for a predetermined period of time. A device such as an auger blends the sludge.
In U.S. Pat. No. 4,226,712 a liquid waste is treated with lime to kill pathogens and partially dry the substance within an initial reaction chamber. The resulting substance is subsequently discharged to a secondary chamber which constitutes a drying kiln surrounded by a furnace which produces the heat for the drying process. In a preferred embodiment, an auger or screw-type conveyor is utilized to move the liquids from the lime treatment reactor to the drying kiln.
U.S. Pat. No. 6,776,914 concerns a process for treating liquid septage or other biological waste in which an alkaline material is added to a volume of wastewater in order to raise the pH of the mixture. The alkaline/wastewater mixture is subsequently stored for a period of time before it is subjected to further treatment methods including flocculation, de-watering, and pasteurization, at which point, the material may be used as fertilizer without requiring further permits.
In U.S. Pat. No. 5,275,733, a process can stabilize a dewatered wastewater sludge with an acidic or basic substance that creates a slightly elevated pH condition within the sludge material. This treatment stabilizes the sludge which reduces pathogens and odors, but allows for the rapid maturation of microbial activity in subsequent treatment processes.
U.S. Pat. No. 6,808,305 concerns preparing a sludge or other biological waste material for dewatering by mixing additives with the sludge utilizing an in-line power-blending device. This method entails the introduction of a stream of fluid additive such as an adhesive to flowing sewage sludge or other mixture. The device is to be fed by a sludge-pumping device which pumps sludge into the blending device. An additive is also pumped into the device at a pre-measured rate. The sludge and additive are then forced through a chamber containing an impeller blade which thoroughly blends the two constituents. The mixture is then discharged from the device to the dewatering equipment.